1. Field of the Invention
The present invention relates to a plasma processing method and, more particularly, a method of keeping plasma under a predetermined state or changing it to the predetermined state on the basis of spectral strengths of its emission light.
2. Description of the Related Art
When a plurality of objects such as semiconductor wafers are successively etching-processed at the same process in the plasma etching apparatus, for example, plasma generating parameters such as RF power, the vacuum degree in a process chamber and flow rates of processing gases are set constant. Even if these parameters are set constant for this etching apparatus, however, practical etching conditions change as time goes by. This is because conditions in the process chamber are changed by by-products such as a film deposited in the chamber as time goes by.
The process chamber must be therefore periodically cleaned, and when the process which is applied to the wafers becomes more precise, the process chamber must be more frequently cleaned. Even if the process chamber is cleaned in this manner, conditions in it just after it is cleaned become different from those in it just before it is cleaned. Even if values of the parameters are made constant, therefore, the state of plasma is changed every etching process by those conditions in the chamber which change as time goes by. This sets a limit to that the precise process can be uniformly applied to every object to be processed.
Further, the state of plasma is sometimes changed even in one time etching process by fluctuation of the frequency of RF power source or the composition of gas, or temperature rise of an electrode or the chamber wall.
As described above, the state of plasma is changed every etching process and every time plural different processes are conducted in the chamber. When the state of plasma is changed in this manner, it becomes difficult to produce semiconductor devices, such as memories of 64M, 256M and more, which require a precise process. Recently, the semiconductors have been more and more highly microstructured. Therefore, the state of plasma must be more precisely controlled.
The state of plasma has been monitored to detect the end point of the etching process. The emission light spectral intensity of a specific wavelength in the plasma is detected in this case. This specific wavelength is selected from wavelengths whose spectral intensity varies depending on the progress of the etching process, and is generally an emission light wavelength of one of those atoms and molecules which form the object etched and the product generated in the etching process. Emission light spectral intensities of plural specific wavelengths are also detected to prevent the end point of the etching process from being wrongly detected because of the drift of plasma. These teachings are disclosed by Published Examined Japanese Patent Application No. 57-12529 and Published Unexamined Japanese patent Application No. 56-133466, for example.
However, it has not been conducted that the state of plasma is controlled while monitor it. Values of parameters, which determine the state of plasma, are kept constant during a plasma process.